1. Field of the Invention
The present invention relates to an adsorbent material capable of selectively and efficiently separating and recovering cesium or strontium in seawater, and a method for producing a crystalline silicotitanate capable of being suitably used for the adsorbent material.
2. Description of the Related Art
As a process technology of wastewater containing radioactive substances, a coprecipitation process is conventionally known (see Patent Literature 1: Japanese Patent Laid-Open No. 62-266499). For radioactive cesium and radioactive strontium which are water-soluble, however, the coprecipitation process is not effective, and the adsorptive removal using an inorganic adsorbent material such as zeolite is carried out at present (see Patent Literature 2: Japanese Patent Laid-Open No. 2013-57599).
In the case where radioactive cesium and radioactive strontium flow out in seawater, however, there is known such a problem that an increase in the concentration of sodium being a seawater component acts in the direction of suppressing the ion-exchange reaction of cesium with an adsorbent material (see Non Patent Literature 1: JAEA-Research 2011-037).
One of inorganic adsorbent materials whose adsorbability of cesium and/or strontium has been studied so far includes crystalline silicotitanates. As the crystalline silicotitanates, there are known a plurality of ones having different compositions such as ones having a Ti/Si ratio of 1:1, 5:12, and 2:1; and besides, it is known that a crystalline silicotitanate having a Ti/Si ratio of 4:3 exists. Non Patent Literature 2 (ZEOLITES, 1990, Vol. 10, November/December) reports that products 3B and 3C produced by hydrothermal processes using an alkoxide being Ti(OET)4 as a Ti source and a colloidal silica as a Si source have a three-dimensional 8-membered ring structure from their X-ray diffraction patterns, and that crystalline silicotitanates having the structure ideally have compositions represented by M4Ti4Si3O16 (M is Na, K and the like), and designates the crystalline silicotitanates having the structure as Grace titanium silicate (GTS-1). Further Non Patent Literature 3 (Keiko Fujiwara, “Modification and Evaluation of Microporous Crystals as Heat Pump Adsorbent Materials” (in Japanese), searched online <URL: http://kaken.nii.ac.jp/pdf/2011/seika/C-19/15501/21560846seika.pdf> on Mar. 3, 2014) describes the effect that a crystalline silicotitanate having a Ti/Si ratio of 4:3 is produced by subjecting a mixed solution having a Ti/Si ratio of 0.32 to a hydrothermal synthesis process. The Non Patent Literature describes the effect that the synthesized crystalline silicotitanate has a strontium ion-exchange capacity.
Although it is conventionally reported as described above that a crystalline silicotitanate having a Ti/Si ratio of 4:3 has a strontium exchange capacity, a further improvement of the adsorption performance of strontium and cesium is desired. Therefore, an object of the present invention is to provide an adsorbent material high in the adsorption performance of cesium and strontium also in seawater, and an industrially advantageous method for producing a crystalline silicotitanate effective as the adsorbent material.